The present invention relates to a thermal type flowmeter and, more particularly, to a thermal type flowmeter which has a high resolution of power in sensing a small amount of flow of a liquid and which is also capable of sensing a reverse flow.
While a fluid to be measured flows as a laminar or turbulent current through a stream pipe, a boundary layer of flow is formed along the inner wall surface of the stream pipe. When the stream pipe is heated by a heat-sensitive resistance wire wound thereon, the heat is carried over into the fluid by heat conduction through the stream pipe wall and through the boundary layer of the fluid's flow. The heat conductivity of the stream pipe is determined as a variable value depending upon the composition of its material and the amount of heat transferred through its surface, and the heat conductivity of the boundary layer of the fluid's flow is determined as a value relating to the specific amount of heat in the fluid to be measured. Accordingly, the heat conductivity of the stream pipe is calculated from such data as its make-up (i.e. the material it is made of), the pipe's diameter etc. On the other hand, the heat conductivity of the fluid, is determined by its density and flow rate. The thermal-type flowmeter can be used as a simple mass flowmeter which, being based upon the above-mentioned operating principle, can determine the mass flow of a fluid of a known kind without hindering the fluid from flowing.
A conventional thermal type flowmeter comprises a heat-conductive stream pipe provided with an upstream side and a downstream side temperature-sensing element having resistance wires wound round the pipe at a certain distance there-between to assure that there will be no thermal influence with each other, and a control means to control a bridge circuit including a ground point at one end and a connecting point at the other end of each of the temperature-sensing elements so as to keep the temperature difference between the upstream side and the downstream side temperature-sensing elements at a constant level, and which determines a mass flow of the fluid from the voltage measured at the connecting point of the downstream side temperature-sensing element.
However the conventional thermal type flowmeter as mentioned above has a non-linear relationship between the flow rate and the sensor output, which show a small inclination and an inflection point in the small range of flow wherein the flow rate approaches zero thereby indicating the decreased sensitivity of the sensor output in the small range.